I found a way to export the certificate: I cannot post it here because it is not possible to attach a file in this forum but I'm going to explain you the steps to extract it here:

1-Install manually NetExtender on a machine Windows Vista/Windows 7 where you never installed it before. (you can use a virtual machine so you can revert back to a vanilla situation to do some testing)

2-When you are prompted to install the device driver mark the checkbox "Always trust software from SonicWALL inc." and then click on Install

3- When the installation is complete open the Certificate Manager issuing the command certmgr.msc from the command line or from the Run menu

the command to install it silently if you have the MSI is MSIEXEC.EXE /I <the msi file> /qn /norestart ALLUSERS=2

But there is a problem: if the software was never installed before on the machine ( >= Vista) the silent installation will not be totally silent due to the fact that the installer want to deploy a device driver signed by SonicWall.

Deploying that certificate the OS will not ask the user intervention to approve the installation of the device driver and your installer will really run silently.

But have a look to the note I wrote: if the software was never installed before on the machine ( >= Vista) the silent installation will not be totally silent due to the fact that the installer want to deploy a device driver signed by SonicWall.